Popular action that's brave for sure (6)
I believe the answer is:
indeed
'sure' is the definition.
(I've seen this before)
'popular action' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'action' becomes 'deed' (deed is a kind of action).
'in'+'deed'='INDEED'
'that's brave for' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for indeed that I've seen before include "Word expressing surprise, disbelief, etc" , "Certainly, quite so" , "Yes, truthfully" , "Truthfully, really" , "beyond doubt" .)